Cobaloximes with dimesitylglyoxime: Synthesis, characterization, and spectral correlations with the related cobaloximes

Article abstract of DOI:10.1021/om049096m

Alkyl and non-alkyl cobaloximes with dimesitylglyoxime have been synthesized and characterized with ¹H and ¹³C NMR, UV-vis, and X-ray diffraction. The X-ray structures of MeCo(dmestgH)₂Py, ClCo(dmestgH)₂Py, and BrCo(dmestgH)₂Py are reported. The cis-trans influence has been studied by ¹H and ¹³C NMR, UV-vis, and X-ray diffraction and is correlated with the reported cobaloximes. The spectral correlations are much better understood when both cobalt anisotropy and ring current are considered operating together. The trans influence of R/X has been monitored by the coordination shift of the Py_γ proton/carbon, and its chemical shift is a net result of the interplay of cobalt anisotropy and the trans effect of the R/X group. Two factors have been considered to study cis influence: (a) the effect of axial ligands on the equatorial dioxime moiety and (b) the effect of dioxime on the axial ligands. It is found that C=N and Py_α are the most sensitive to any change in the molecule. A change in the axial R/X and dioxime moieties affects the C=N resonance, whereas Py_α is sensitive to the change in R/X (trans effect) and the ring current of the dioxime (cis influence). A good correlation between δ(¹³C, C=N) and Δδo( ¹H, Py_α) suggests the presence of ring current throughout the Co(dioxime) metallabicycle, and the negative slope indicates that they are effected in opposite directions. It is found that dmestgH complexes have the maximum cis influence among all the reported cobaloximes. A cyclic voltammetry study for both alkyl and non-alkyl cobaloximes is reported. The reduction from Co(III) to Co(II) and from Co(II) to Co(I) is found to be more difficult in ClCo(dmestgH)₂Py as compared to the other chlorocobaloximes (gH, dmgH, dpgH).

Full text of DOI:10.1021/om049096m

Organometallics 2005, 24, 1501-1510
1501
Cobaloximes with Dimesitylglyoxime: Synthesis,
Characterization, and Spectral Correlations with the
Related Cobaloximes†
Debaprasad Mandal and B. D. Gupta*
Department of Chemistry, Indian Institute of Technology, Kanpur, India 208016
Received November 22, 2004
Alkyl and non-alkyl cobaloximes with dimesitylglyoxime have been synthesized and
characterized with 1H and 13C NMR, UV-vis, and X-ray diffraction. The X-ray structures of
MeCo(dmestgH)2Py, ClCo(dmestgH)2Py, and BrCo(dmestgH)2Py are reported. The cis-trans
1
influence has been studied by H and 13C NMR, UV-vis, and X-ray diffraction and is
correlated with the reported cobaloximes. The spectral correlations are much better
understood when both cobalt anisotropy and ring current are considered operating together.
The trans influence of R/X has been monitored by the coordination shift of the Pyγ proton/
carbon, and its chemical shift is a net result of the interplay of cobalt anisotropy and the
trans effect of the R/X group. Two factors have been considered to study cis influence: (a)
the effect of axial ligands on the equatorial dioxime moiety and (b) the effect of dioxime on
the axial ligands. It is found that CdN and PyR are the most sensitive to any change in the
molecule. A change in the axial R/X and dioxime moieties affects the CdN resonance, whereas
PyR is sensitive to the change in R/X (trans effect) and the ring current of the dioxime (cis
influence). A good correlation between δ(13C, CdN) and ∆δ(1H, PyR) suggests the presence
of ring current throughout the Co(dioxime) metallabicycle, and the negative slope indicates
that they are effected in opposite directions. It is found that dmestgH complexes have the
maximum cis influence among all the reported cobaloximes. A cyclic voltammetry study for
both alkyl and non-alkyl cobaloximes is reported. The reduction from Co(III) to Co(II) and
from Co(II) to Co(I) is found to be more difficult in ClCo(dmestgH)2Py as compared to the
other chlorocobaloximes (gH, dmgH, dpgH).
Introduction
on cobalamins suggests that the structural effects of
changes in R are similar to those found in cobaloximes
and sometimes can be related to their chemical behav-
ior.4 The studies on cobaloximes have furnished some
insight into the factors that affect homolysis of the
Co-C bond and have allowed in-depth analysis of the
variation in the geometry of the R-Co-B fragment (in
terms of electronic and steric properties of R and
B).2,3f,g,h Most of the studies in the recent past have
described the spectral and structural properties of
cobaloximes. Despite this wealth of information a great
deal of interest has been devoted to the study of
correlations between the NMR spectral data and mo-
The chemistry and molecular structure of bis(di-
methylglyoximato)cobalt(III) complexes, trivially known
as cobaloximes,1 have been of great interest to chemists
for the past four decades for two reasons. First, the co-
ordination chemistry of these complexes is far-reaching,
with almost unlimited possibilities for substituents in
the axial position and variation in the equatorial
ligands.2 Second, many organometallic cobaloxime de-
rivatives have been used as model compounds for the
study of vitamin B12 coenzyme.2
Many different approaches to qualitatively rationalize
the trends in structure, NMR, and thermodynamic and
kinetic properties as a function of steric and electronic
effects of the axial ligands in cobaloximes have been
reported.3 The recently available crystallographic data
(3) (a) Marzilli, L. G.; Bayo, F.; Summers, M. F.; Thomas, L. B.;
Zangrando, E.; Bresciani-Pahor, N.; Mari, M.; Randaccio, L. J. Am.
Chem. Soc. 1987, 109, 6045. (b) Brown, K. L.; Lyles, D.; Penencovici,
M.; Kallen, R. G. J. Am. Chem. Soc. 1975, 97, 7338. (c) Randaccio, L.;
Bresciani-Pahor, N.; Toscano, P. J.; Marzilli, L. G. J. Am. Chem. Soc.
1981, 103, 6347. (d) Charland, J. P.; Zangrando, E.; Bresciani-Pahor,
N.; Randaccio, L.; Marzilli, L. G. Inorg. Chem. 1993, 32, 4256. (e) Cini,
R.; Moore, S. J.; Marzilli, L. G. Inorg. Chem. 1998, 37, 6890. (f)
Bresciani-Pahor, N.; Randaccio, L.; Zangrando, E.; Summers, M. F.;
Ramsden, J. H., Jr.; Marzilli, P. A.; Marzilli, L. G. Organometallics
1985, 4, 2086. (g) Toscano, P. J.; Swider, T. F.; Marzilli, L. G.;
Bresciani-Pahor, N.; Randaccio, L. Inorg. Chem. 1983, 22, 3416. (h)
Brown, K. L.; Satyanarayana, S. J. Am. Chem. Soc. 1992, 114, 5674.
(i) Zangrando, E.; Bresciani-Pahor, N.; Randaccio, L.; Charland, J. P.;
Marzilli, L. G. Organometallics 1986, 5, 1938. (j) Drago, R. S. Inorg.
Chem. 1995, 34, 3543. (k) Drago, R. S. J. Organomet. Chem. 1996,
512, 61.
* To whom correspondence should be addressed. Tel: +91-512-
2597046. Fax: +91-512-2597436. E-mail: bdg@iitk.ac.in.
† Dedicated to Prof. C. N. R. Rao, JNCASR, Bangalore, India.
(1) Cobaloximes have the general formula RCo(L)2B, where R is an
organic group σ-bonded to cobalt, B is an axial base trans to the organic
group, and L is a monoanionic dioxime ligand (e.g. glyoxime (gH),
dimethylglyoxime (dmgH), 1,2-cyclohexanedione dioxime (chgH), di-
phenyl glyoxime (dpgH), and dimesitylglyoxime (dmestgH)).
(2) (a) Toscano, P. J.; Marzilli, L. G. Prog. Inorg. Chem. 1985, 31,
105. (b) Bresciani-Pahor, N.; Forcolin, M.; Marzilli, L. G.; Randaccio,
L.; Summers, M. F.; Toscano, P. J. J. Coord. Chem. Rev. 1985, 63, 1.
(c) Randaccio, L.; Bresciani-Pahor, N.; Zangrando, E.; Marzilli, L. G.
Chem. Soc. Rev. 1989, 18, 225. (d) Randaccio, L. Comments Inorg.
Chem. 1999, 21, 327 and references therein. (e) Gupta, B. D.; Roy, S.
Inorg. Chim. Acta 1988, 146, 209.
(4) (a) Randaccio, L.; Furlan, M.; Geremia, S.; Slouf, M.; Srnova, I.;
Toffoli, D. Inorg. Chem. 2000, 39, 3403. (b) Randaccio, L.; Geremia,
S.; Nardin, G.; Slouf, M.; Srnova, I. Inorg. Chem. 1999, 38, 4087.
10.1021/om049096m CCC: $30.25 © 2005 American Chemical Society
Publication on Web 02/12/2005

1502 Organometallics, Vol. 24, No. 7, 2005
Mandal and Gupta
lecular structures of these complexes.2,5 The driving
force behind this work is to obtain a clear relationship
between all the properties. This would help to system-
atize the large amount of chemical information currently
available, and it might lead to a successful design of
novel cobaloximes with desired properties. Several
studies with this aim have appeared in the litera-
Chart 1
1
ture.3,6,7 For instance, the reported trends in H NMR
chemical shifts in cobaloximes (R/X)Co(dmgH)2B have
been related to the mutual cis and trans influence of
the axial ligands.3,6 The study also includes the multi-
linear correlation of 1H NMR chemical shifts of B with
the Co-dioxime charge-transfer band.6 These spectral
correlations initially were interpreted by Marzilli et al.
on the basis of cobalt anisotropy,3d,8 but recently Lo´pez
et al. invoked the ring current formalism.6 As per this
model, ring current resulting from the 12-π-delocalized-
electron system of cobaloximes (8 electrons from CdN
and 4 from Co) would affect the nuclei in different ways,
depending upon their relative position to the metalla-
bicycle, shielding those on the top and deshielding those
at the sides of the ring. Most of the information has
come from the study of cobaloximes with dmgH as the
equatorial ligand, and studies involving other dioximes
such as gH,9a,b chgH,9c and dpgH9d or mixed dioximes9e,f
are few. Most of the correlations have been derived from
1H NMR studies, and 13C NMR studies have been done
Experimental Section
Glyoxime and alkyl halides were purchased from Aldrich
Chemical Co. and were used as received. Silica gel (100-200
mesh) and distilled solvents were used in all chromatographic
separations. Dichloroglyoxime, dimesitylglyoxime, chlorocobal-
oxime24 were synthesized according to the literature proce-
dure.11
1H and 13C NMR spectra were recorded on a JEOL JNM
1
LAMBDA 400 FT NMR instrument (at 400 MHz for H and
at 100 MHz for 13C) in CDCl3 solution with TMS as internal
standard. NMR data are reported in ppm. UV-vis spectra
were recorded on a JASCO V570 spectrophotometer in dry
chloroform at 298 K. Elemental analysis was carried out at
the Regional Sophisticated Instrumentation Center, Lucknow,
and at IIT Kanpur. A Julabo UC-20 low-temperature refriger-
ated circulator was used to maintain the desired temperature.
Cyclic voltammetry measurements were carried out using a
BAS Epsilon electrochemical workstation with a platinum
working electrode, a Ag/AgCl reference electrode (3 M KCl),
and a platinum-wire counter electrode. All the measurements
were performed in 0.1 M nBu4NPF6 in dichloromethane (dry),
at a concentration of 1 mM of each complex. In addition, in a
separate series of experiments, an internal reference system
(ferrocene/ferrocenium ion) was used. Under the conditions
used, the reversible Fc/Fc+ potential occurred at 0.51 V vs the
Ag/AgCl electrode.
on a few complexes only. Efforts to correlate 1H and 13
C
NMR resonances were made, but the results were rather
poor.10
Each of the two models, cobalt anisotropy and ring
current, has some shortcomings and do not explain the
existing data properly when used in isolation from each
other.
We have, therefore, undertaken this study on (R/X)Co-
(dmestgH)2Py (see Chart 1). All of the compounds except
for 6 are new. The X-ray structures of 1, 6, and 7 are
reported. The cis-trans influence has been studied by
1H and 13C NMR, UV-vis, and X-ray. The aim of the
present study is (a) to rationalize/modify the existing
models, (b) to see if 13C gives similar or better informa-
tion than 1H NMR and if there is any correlation in 1H
and 13C resonances, (c) to verify if the trends obtained
in dmgH complexes can be extended to other dioxime
complexes, and (d) to see if X-ray gives any information
on the cis-trans influence.
X-ray Structural Determination and Refinement.
Orange crystals were obtained by slow evaporation of the
solutions of 1, 6, and 7 in dichloromethane/acetonitrile. Single-
crystal X-ray data were collected at room temperature for 1
and 6 and at 100 K for 7 on a Bruker SMART APEX CCD
diffractometer using graphite-monochromated Mo KR radia-
tion (λ ) 0.710 73 Å). The linear absorption coefficients,
scattering factors for the atoms, and anomalous dispersion
corrections were taken from ref 12a. The data integration and
reduction were processed with SAINT13 software. An empirical
absorption correction was applied to the collected reflections
with SADABS14 using XPREP.15 The structure was solved by
direct methods using SHELXTL16 and was refined on F2 by
the full-matrix least-squares technique using the SHELXL-
9712b program package. All non-hydrogen atoms were refined
anisotropically. The hydrogen atom positional and thermal
parameters were not refined but were included in the structure
factor calculations. The crystal data for the three structures
are collected in Table 1.
(5) Bresciani-Pahor, N.; Geremia, S.; Lo´pez, C.; Randaccio, L.;
Zangrando, E. Inorg. Chem. 1990, 29, 1043.
(6) (a) Lo´pez, C.; Alvarez, S.; Solans, X.; Font-Altaba, M. Inorg.
Chim. Acta 1986, 111, L19. (b) Lo´pez, C.; Alvarez, S.; Solans, X.; Font-
Altaba, M. Inorg. Chem. 1986, 25, 2962. (c) Gilaberte, J. M.; Lo´pez,
C.; Alvarez, S.; Font-Altaba, M.; Solans, X. New. J. Chem. 1993, 17,
193.
(7) Gupta, B. D.; Qanungo, K. J. Organomet. Chem. 1997, 543, 125.
(8) Moore, S. J.; Marzilli, L. G. Inorg. Chem. 1998, 37, 5329.
(9) (a) Gupta, B. D.; Yamuna, R.; Singh, V.; Tewari, U. Organo-
metallics 2003, 22, 226 and references therein. (b) Lo´pez, C.; Alvarez,
S.; Solans, X.; Font-Altaba, M. Inorg. Chim. Acta 1986, 121, 71. (c)
Gupta, B. D.; Qanungo, K.; Yamuna, R.; Pandey, A.; Tewari, U.; Singh,
V.; Vijaikanth, V.; Barclay, T.; Cordes, W. J. Organomet. Chem. 2000,
608, 106. (d) Lo´pez, C.; Alvarez, S.; Font-Bardia, M.; Solans, X. J.
Organomet. Chem. 1991, 414, 245. (e) Gupta, B. D.; Singh, V.; Yamuna,
R.; Barclay, T.; Cordes, W. Organometallics 2003, 22, 2670. (f) Gupta,
B. D.; Yamuna, R.; Singh, V.; Tewari, U.; Barclay, T.; Cordes, W. J.
Organomet. Chem. 2001, 627, 80.
RCo(dmestgH)2Py (1-5). These compounds were synthe-
sized by the general procedure detailed earlier for the synthesis
(11) Lance, K. A.; Goldsby, K. A.; Busch, D. H. Inorg. Chem. 1990,
29, 4537.
(12) (a) International Tables for X-ray Crystallography; Kynoch
Press: Birmingham, England, 1974; Vol. IV. (b) Sheldrick, G. M.
SHELXL-97: Program for Crystal Structure Refinement; University
of Go¨ttingen, Go¨ttingen, Germany, 1997.
(13) SAINT+, 6.02 ed.; Bruker AXS, Madison, WI, 1999.
(14) Sheldrick, G. M. SADABS, Empirical Absorption Correction
Program; University of Go¨ttingen, Go¨ttingen, Germany, 1997.
(15) XPREP, 5.1 ed.; Siemens Industrial Automation, Madison, WI,
1995.
(16) Sheldrick, G. M. SHELXTL Reference Manual: Version 5.1;
Bruker AXS: Madison, WI, 1997.
(10) (a) Moore, S. J.; Lachicotte, R. J.; Sullivan, S. T.; Marzilli, L.
G. Inorg. Chem. 1999, 38, 383. (b) Stewart, R. C.; Marzilli, L. G. Inorg.
Chem. 1977, 16, 424. (c) Kargol, J. A.; Crecely, R. W.; Burmeister, J.
L.; Toscano, P. J.; Marzilli, L. G. Inorg. Chim. Acta 1980, 40, 79.

Cobaloximes with Dimesitylglyoxime
Organometallics, Vol. 24, No. 7, 2005 1503
Table 1. Crystal Data and Structure Refinement Details for 1, 6, and 7
[MeCo(dmestgH)2Py]‚CH2Cl2
[ClCo(dmestgH)2Py]‚CH3CN
[BrCo(dmestgH)2Py]‚CH2Cl2
empirical formula
formula wt
temp (K)
radiation, λ (Å)
cryst syst
space group
unit cell dimens
a (Å)
C94H112Cl4Co2N10O8
1769.60
C94H108Cl2Co2N12O8
1722.68
C92H106Br2Cl4Co2N10O8
1895.35
293(2)
293(2)
100(2)
Mo KR, 0.710 73
monoclinic
C2/c
Mo KR, 0.710 73
monoclinic
C2/c
Mo KR, 0.710 73
monoclinic
C2/c
27.939(15)
9.226(5)
37.232(2)
90.000(0)
27.776(8)
9.071(2)
36.827(10)
90.000(0)
27.579(17)
9.116(5)
36.610(2)
90.000(0)
b (Å)
c (Å)
R (deg)
â (deg)
108.006(17)
90.000(0)
108.043(6)
90.000(0)
108.493(2)
90.000(0)
γ (deg)
V (Å3)
9127(5)
8822(5)
8729(7)
Z
4
4
4
F(calcd) (Mg/m3)
1.288
1.297
1.445
µ (mm-1
F(000)
)
0.541
0.500
1.481
3728
3632
3936
cryst size (mm3)
index ranges
0.22 × 0.15 × 0.13
-37 e h e 31
-12 e k e 12,
-28 e l e 49
29 710
0.25 × 0.17 × 0.14
-26 e h e 37
-12 e k e 11
-49 e l e 46
28 547
0.26 × 0.18 × 0.15
-36 e h e 36
-12 e k e12
-48 e l e 26
28 518
no. of rflns collected
no. of indep rflns
refinement method
GOF on F2
11 254
10 846
10 790
full-matrix least squares on F2
1.017
full-matrix least squares on F2
1.003
full-matrix least squares on F2
1.053
final R indices (I > 2σ(I))
R1 ) 0.0689
wR2 ) 0.1621
R1 ) 0.1014
wR2 ) 0.1806
11 254/0/545
R1 ) 0.0837
wR2 ) 0.1683
R1 ) 0.1634
wR2 ) 0.2004
10 846/0533
R1 ) 0.0727
wR2 ) 0.2119
R1 ) 0.0935
wR2 ) 0.2268
10 790/0/544
R indices (all data)
no. of data/restraints/params
Table 2. Elemental Analysis Data for 1-9
of RCo(dioxime)2Py7 and involved the reaction of cobaloxime(I)
with organic halide. In a typical procedure, N2 gas was bubbled
for 10 min with stirring through a solution of ClCo(dmestgH)2Py
(0.100 g, 0.120 mmol) in about 50 mL of ethanol at -10 °C.
The solution turned blue after the addition of a few drops of
aqueous NaOH followed by a solution of sodium borohydride
(0.05 g, 1.2 mmol) in water. The solution immediately turned
orange on the addition of methyl iodide (0.07 g, 0.50 mmol) in
ethanol (1 mL). Stirring was continued at 0 °C for 1 h. The
reaction mixture was poured into 20 mL of water containing
a few drops of pyridine. The resulting orange precipitate was
filtered, washed with water, and dried. The product was
purified on a silica gel column using dichloromethane/
petroleum ether (5/1) followed by dichloromethane. Yield:
0.087 g, 90%.
XCo(dmestgH)2Py (6-9). These compounds were synthe-
sized by the substitution of the chloride group in ClCo-
(dmestgH)2Py (6) by another inorganic group X-.9a,17 In a
typical experiment, a solution of sodium azide (0.079 g, 1.22
mmol) in 1 mL of water was added to a refluxing suspension
of ClCo(dmestgH)2Py (0.200 g, 0.24 mmol) in 30 mL of
methanol. The reaction mixture was further refluxed for 4 h.
The solution was evaporated to dryness. The solid was dis-
solved in a minimum volume of CHCl3 and loaded on the silica
gel column. The compound N3Co(dmestgH)2Py (9) was eluted
out with a mixture of 5-10% CHCl3 in ethyl acetate. Yield:
(0.145 g, 72%).
% found (calcd)
no.
formula
C
H
N
1
2
3
4
5
6
7
8
9
C
C
C
C
C
C
C
C
C
46H54CoN5O4
47H56CoN5O4
48H58CoN5O4
49H60CoN5O4
48H58CoN5O4
69.14 (69.07) 6.78 (6.80) 8.79 (8.76)
69.35 (69.36) 6.90 (6.93) 8.64 (8.60)
69.67 (69.63) 7.02 (7.06) 8.50 (8.46)
69.86 (69.90) 7.16 (7.18) 8.29 (8.32)
69.60 (69.63) 7.01 (7.06) 8.42 (8.46)
45H51ClCoN5O4 65.91 (65.89) 6.23 (6.27) 8.50 (8.54)
45H51BrCoN5O4 62.46 (62.50) 5.90 (5.94) 8.08 (8.10)
45H51CoN6O6
45H51CoN8O4
65.00 (65.05) 6.18 (6.19) 10.08 (10.11)
65.33 (65.36) 6.18 (6.22) 13.58 (13.55)
the addition of Et3N. However, when we tried to prepare
it using the conventional procedure9c,e reported for ClCo-
(dioxime)2Py (dioxime ) gH, dmgH, dpgH) we could get
a maximum yield of 10%. The side product was EPR
• 11,18
active and looked like the radical Co-O2 .
No
attempt was made to analyze this.
Inorganic cobaloximes, XCo(dioxime)2Py (X ) Br, N3,
NO2) have been well described in the literature.9a In
general, two methods have been used for their prepara-
tion (a) by aerial oxidation of the stoichiometric mixture
of reactants9b and (b) by substitution of chloride in
chlorocobaloxime by another nucleophile.9a,17 We have
used method b for the synthesis of XCo(dmestgH)2Py.
The synthesis of RCo(dmestgH)2Py was accomplished
by a slight modification of the well-established proce-
dure for RCo(dioxime)2Py complexes. The cobaloxime(I)
anion was generated in situ by NaBH4 reduction of the
preformed chlorocobaloxime followed by oxidative alkyl-
ation.2e We found that ethanol was a better solvent than
methanol. A large excess of ethanol and a 10-fold excess
of NaBH4 was essential; otherwise, the yield was poor
and the starting complex ClCo(dmestgH)2Py was recov-
ered.
Results and Discussion
Synthesis. We have synthesized two series of com-
plexes, RCo(dmestgH)2Py (1-5) and XCo(dmestgH)2Py
(6-9). All of the complexes except for 6 are new.
Elemental analysis data for 1-9 are given in Table 2.
ClCo(dmestgH)2Py was synthesized according to the
procedure by Busch et al.11 The preparation requires
(17) Gupta, B. D.; Tewari, U.; Barclay, T.; Cordes, W. J. Organomet.
Chem. 2001, 629, 83.
(18) Schrauzer, G. N.; Lee, L. P. J. Am. Chem. Soc. 1970, 92, 1551.

1504 Organometallics, Vol. 24, No. 7, 2005
Table 3. 1H NMR Data for 1-9 in CDCl3
Mandal and Gupta
a
Py
mesityl group
Me (4 and 6)
no.
R (d)
â (t)
7.41
γ (t)
O-H‚‚‚O
Me (2)
1.51
aromatic(s)
Co-CH2
rest of alkyl chain
1
8.99
7.88
18.94
2.16
2.19
2.16
2.22
2.17
2.20
2.17
2.21
2.15
2.27
2.19
2.25
2.19
2.29
2.14
2.19
2.20
2.26
6.61
6.73
6.60
6.73
6.60
6.73
6.61
6.73
6.59
6.73
6.65
6.78
6.64
6.78
6.64
6.78
6.65
6.80
1.45
2
3
4
5
6
7
8
9
8.95
8.95
8.96
8.98
8.66
8.66
8.68
8.66
7.40
7.40
7.40
7.36
7.34
7.34
7.39
7.36
7.86
7.86
7.86
7.82
7.87
7.87
7.91
7.88
18.75
18.77
18.76
18.68
18.82
18.83
18.51
18.62
1.45
1.45
1.45
1.45
1.55
1.54
1.49
1.51
2.42
2.30
2.32
2.66
0.94 (t)
0.95 (t), 1.56
0.91 (t), 1.40 (q), 1.62 (m)
1.02 (d)
a ClCo(dmestgH)2(morpholine): mesityl group (Me) 2.30, 2.22, 2.24 ppm, aromatic 6.79, 6.81 ppm; morpholine 3.82 (d, J ) 10.8 Hz),
3.31 (t, J ) 13.2 Hz), 2.58 ppm (q, J ) 10.4 Hz).
a
Table 4. 13C NMR Data for 1-9 in CDCl3
Py
no. CdN
R
â
γ
mesityl (Me)
Co-CH2
aromatic
others
1
2
3
4
5
6
7
8
9
152.33 151.03 124.94 138.33 20.00, 20.43, 21.04
152.46 151.16 125.00 138.27 19.93, 20.34, 21.03
152.45 151.12 124.99 138.25 19.90, 20.21, 21.02
152.43 151.13 124.95 138.21 21.00, 20.25, 19.92
27.72
26.50
37.91
33.40
42.25
126.62, 128.28, 128.58, 137.00, 137.83, 138.69
126.62, 128.32, 128.62, 137.11, 137.82, 138.70
126.62, 128.31, 128.63, 137.12, 137.77, 138.70
126.63, 128.30, 128.61, 137.12, 137.77, 138.66
126.63, 128.40, 128.67, 137.26, 138.12,138.62, 137.84 27.37
126.11, 128.10, 128.87, 136.60, 139.60, 139.12, 139.26
126.17, 128.09, 128.90, 139.55, 136.68, 139.23, 139.28
125.83, 128.14, 128.94, 136.54, 138.63, 139.49, 138.90
126.00, 128.16, 128.86, 136.57, 138.61, 139.06, 139.40
16.64
28.45, 22.62
32.61, 24.03, 13.95
153.02 151.26 124.82
155.57 152.06 125.26
156.00 151.74 125.24
155.75 151.56 125.49
155.52 152.05 125.39
b
b
b
b
b
20.40, 20.89, 20.95
19.94, 20.28, 21.10
20.03, 20.45, 21.06
19.94, 20.27, 21.08
19.75, 19.92, 21.07
a ClCo(dmestgH)2(morpholine): CdN 155.99 ppm; mesityl group (Me) 20.37, 21.08, 21.27 ppm; aromatic 125.89, 128.32, 129.28, 136.20,
139.51, 139.57 ppm; morpholine 49.85, 68.30 ppm. b Merge with aromatic carbons.
Spectroscopy: Characterization of the Com-
Chart 2. Numbering Scheme and Ring Current
Effect of the Metallabicycle and Pyridine Ring
plexes. The free ligand dmestgH2 has been fully char-
1
acterized by Busch et al. 13C NMR values (but not H)
for ClCo(dmestgH)2Py were reported but not assigned.11
1
There is a drastic change in H NMR when dmestgH2
is coordinated to cobalt in complexes 1-9; for example,
both of the o-methyl groups in uncoordinated dmestgH2
are equivalent and appear at 2.14 ppm, whereas these
are nonequivalent in 1-9. This is due to restricted
rotation around the C-C bond between oximinic carbon
and phenyl carbon. One of the three methyl groups of
mesityl group at 2-position is close to axial pyridine ring
(see X-ray details later) and is highly shielded by the
ring current and appears at around 1.50 ppm. This is
confirmed by 1H NMR spectrum of ClCo(dmestgH)2-
(morpholine). Here, morpholine lacks ring current and
hence no shielding to the methyl at 2-position occurs
and thus it appears close to the other methyl at 4- and
6-positions (see footnote in Table 3). The chemical shift
of the methyl group at the 6-position is affected by the
nature of the axial R/X group. However, the shift is not
that large. The methyl at the 4-position is not affected
at all and remains constant. Because of the restricted
rotation of the mesityl group, two aromatic hydrogens
of the mesityl group also are nonequivalent in 1-9 and
occur downfield compared to the free ligand.
are consistent with the related dioxime compounds
previously described.7 On the other hand, 13C chemical
shifts have been assigned for only a few cobaloximes
until recently. Further comments are, therefore, war-
ranted on these assignments.
For RCo(dmestgH)2Py (1-5), apart from the axial
organic group, five sets of 13C resonances for PyR, Pyâ
Pyγ, CdN, and the mesityl group are observed. These
are assigned on the basis of chemical shifts. PyR and
CdN appear close together and are assigned by DEPT.
All of the 1H and 13C NMR values are given in Tables
3 and 4, respectively, and the numbering scheme is
given in Chart 2. All of these complexes exist as six-
coordinate species in CDCl3 (the solvent used for NMR),
as the cobalt-bound CH3 as well as dmestgH protons
The 1H NMR spectra of the other complexes are easily
assigned on the basis of the chemical shifts. The signals
are assigned according to their relative intensities and

Cobaloximes with Dimesitylglyoxime
Organometallics, Vol. 24, No. 7, 2005 1505
do not shift even on the addition of a large excess of
pyridine to the NMR sample.
Solubility. The orange solid compounds 1-9 are
highly soluble in common organic solvents such as
dichloromethane, chloroform, and THF and are spar-
ingly soluble/insoluble in methanol, ethanol, aceto-
nitrile, and ether.
Cis and Trans Influence. A cis and trans influence
study includes the investigation of all possible steric and
electronic changes detectable in the cis and trans
ligands. Cobaloximes have become the ideal systems to
study these effects in octahedral complexes. In general,
either the axial ligand R/X or base B is varied and
changes in the cis equatorial dioxime moiety are ob-
served or the axial ligands are kept constant and the
equatorial dioxime moiety is varied and changes in the
axial ligands are monitored spectroscopically.
influence should, in fact, be monitored through the
coordination shift (∆δ ) δcomplexes - δfree base) of the
γ-proton only, since the chemical shift of the R-proton
includes not only the trans influence of R/X but also the
cis influence of the dioxime moiety. In the study by
Lo´pez and Marzilli, either the base did not have a
γ-proton or the signal was masked by the solvent peaks
and hence was not considered for study. Besides, the
phenyl ring in dpgH complexes is more than 5 Å away
from the PyR proton and hence should not affect the
chemical shift of PyR. Since 1H NMR operates in a
through-bond as well as through-space manner but 13
C
NMR operates mainly through-bond, any shift in 13C
δ(PyR) will be due to cobalt anisotropy only, whereas
the change in 1H δ(PyR) is affected both by cobalt
anisotropy as well as by ring current of the dioxime.
Both operate in opposite directions. Lo´pez and Marzilli
made no attempt to rationalize this by 13C NMR study.
We have considered the Lo´pez model with some
modification.6a Three factors simultaneously acting on
a particular atom to determine the NMR shift of the
coordinated ligand from the free ligand are (a) cobalt
anisotropy25 (leads to deshielding of all the ligand atoms
through the σ backbone and it decreases with the
distance from the metal), (b) the long-range effect of
magnetic anisotropy such as ring current arising from
the metallabicycle, the axial ligand pyridine, or the
phenyl ring of the mesityl group, and (c) metal to ligand
back-bonding, which may alter the ring current and/or
the cobalt anisotropy.
The chemical shift of Pyγ proton is a net result of the
interplay of cobalt anisotropy and the trans effect of the
R/X group. In the present study the Pyγ proton reso-
nance on coordination to the cobaloximes moiety in 1-5
shifts downfield, and the downfield shift is much larger
in the dpgH and dmestgH complexes (about 0.45 ppm)
than in the corresponding dmgH and chgH complexes
(about 0.25 ppm). The large cobalt anisotropy in dpgH
or dmestgH complexes leads to more electron with-
drawal from pyridine and causes deshielding of the
γ-proton.
(b) 13C Chemical Shift. 13C δ(Pyγ) shifts downfield
on coordination to cobalt, and the downfield shift is
much larger in dpgH and dmestgH complexes as com-
pared to dmgH and chgH complexes. This is similar to
1H NMR information.
1H and 13C NMR studies of RCo(dioxime)2Py (dioxime
) gH, dmgH, chgH, dpgH) have shown that O-H‚‚‚O,
PyR, and CdNoximinic are most affected followed by Pyγ.
The chemical shifts for O-H‚‚‚O resonances in 1-9
are given in Table 3. A comparison of O-H‚‚‚O values
in 1-4 with those in RCo(dioxime)2Py shows a high
downfield shift in 1-4, nearly 0.5-0.7 ppm as compared
to the corresponding dmgH complexes and 1.1-1.2 ppm
as compared to gH complexes. However, the values are
close to those of the dpgH complexes. The O-H‚‚‚O
resonance follows the order dmestgH > dpgH > dmgH
> gH > chgH. This is due to the cis influence of dioxime
on O-H‚‚‚O. A similar order is observed in 6-9. The
value, however, remains almost constant within the
same series in all the complexes. Surprisingly, the
replacement of axial organic R by an inorganic group X
does not lead to any significant shift in O-H‚‚‚O
resonance (compare the values for 1-4 with those for
6-9).
Trans Influence. (a) 1H Chemical Shift. Lo´pez and
Marzilli have monitored the trans influence of the axial
R/X group through the coordination shift of the R-
proton of the axial base ligand in (R/X)Co(dmgH)2B
(B ) 4-tBuPy, 3,5-lutidine, imidazole, 2,6-dimethyl-
pyrazine).3a,d,6,8,10a They have mentioned that the varia-
tion observed in the remaining protons of the base are
small and consequently less affected by the nature of
the trans R/X ligand, in agreement with their larger
distance to the metallabicycle. Also, a different situation
appears for the analogous dpgH complexes, where the
axial ligand signals are not very sensitive to coordina-
tion. This was explained by taking into account the
presence of phenylic rings in the neighborhood of the
axial ligand, producing deshielding of its nuclei.6b We
disagree with their proposal and believe that the trans
1H and 13C NMR studies, however, give conflicting
information on the inorganic cobaloximes. For example,
1
there is no significant shift in the H δ(Pyγ) resonance
whether the axial ligand is R or X (compare 1-5 with
6-9). This may mean that the cobalt anisotropy is
almost same for both R and X. However, the 13C δ(Pyγ)
resonance occurs more downfield (by about 0.7 ppm) in
inorganic cobaloximes as compared to the organo-
cobaloximes (compare 6-9 with 1-5) and this difference
is much larger (about 1.7 ppm) in dmgH and chgH
complexes. This is an expected trend, since the cobalt
anisotropy is larger for X than for R.
Cis Influence. To study cis influence, we have
considered (a) the effect of axial ligands on the equato-
rial dioxime moiety and (b) the effect of dioxime on the
axial ligands. It is found that CdN and PyR are most
sensitive to any change in the molecule. A change in
the axial R/X and dioxime moiety will affect the CdN
resonance, whereas PyR is sensitive to the change in R/X
(trans effect) and the ring current of the dioxime (cis
influence).
CdN. The extent of electron density (i.e. total effect
of cobalt anisotropy and ring current) on Co(dioxime)2
for different dioximes (keeping R/X constant) can be
understood by comparing ∆δ(13C, CdN)26 in R/XCo-
(dioximes)2Py. We find that the δ(13C, CdN) resonance
in 1-9 occurs significantly downfield as compared to
the values in other dioximes (about 3-4 ppm compared
to dmgH and about 5-6 ppm compared to gH com-
plexes). The order based on the upfield shift value of

1506 Organometallics, Vol. 24, No. 7, 2005
Mandal and Gupta
∆δ(13C, CdN) is gH > dmgH > dpgH > chgH >
dmestgH. This means the charge density on CdN in
dmestgH complexes is much lower as compared to
cobaloximes with other dioximes. This is surprising.
This can be rationalized by taking into account the
higher anisotropy in the dpgH and dmestgH complexes.
The variation in R/X group also affects δ(13C, CdN). For
example, it occurs highly upfield (about 2.4 ppm) in 1-5
and downfield in 6-9 (0.7-1.2 ppm) as compared to the
value in uncoordinated demstgH2. This is similar to our
earlier findings in other dioxime complexes. However,
δ(13C, CdN) always occurs upfield from the signal for
the free ligand in the corresponding (R/X)Co(dioximes)2Py
(dioxime ) gH, dmgH, chgH, dpgH) complexes.9
The charge density on CdN should also affect the
O-H‚‚‚O resonance. The higher the charge density on
CdN, the stronger the hydrogen bond, and so a more
upfield shift of O-H‚‚‚O would be expected. This is what
has been observed (see above).
MeCo(dmgH)2Py has partial positive charge on cobalt,
whereas cobalt in ClCo(dmgH)2Py has a partial negative
charge. The pyridine N has partial negative charge and
PyR has positive charge in MeCo(dmgH)2Py, and in
contrast, the reverse is true in ClCo(dmgH)2Py (Sup-
porting Information).
Earlier, Brown and LaRossa have correlated the NQR
(nuclear quadrupole resonance) and NMR results in
cobaloximes based on the electron field gradient (efg)
model.19b In the simplest possible approach to the NQR
studies, they have considered two types of efg: (a) the
combined effect of both axial ligands and (b) the effect
of the planar equatorial dioxime ligand. They found
that, for methylcobaloximes, the average donor strengths
of the methyl group and axial base pyridine put together
are greater than the donor strength of nitrogen in the
planar ligand system, so that the quadruple coupling
constant is negative. In the halocobaloximes, on the
other hand, the average donor strength of the axial
ligands is less than that of the nitrogens in the planar
ligand.
Pyr. A comparison of ∆δ(1H, PyR) in RCo(dioxime)2Py
or XCo(dioxime)2Py (R ) Me, Et, Pr, Bu; X ) Cl, Br,
NO2, N3; dioxime ) gH, dpgH, chgH, dpgH, dmestgH)
shows that it occurs downfield (by 0.4 ppm) in dpgH and
dmestgH complexes as compared to the signals for the
other dioximes. This is as expected, on the basis of the
higher cobalt anisotropy in the latter, but it cannot be
due to the presence of a phenyl ring in dpgH complexes,
as suggested by Lo´pez.
The PyR proton is close to O-H‚‚‚O and therefore
should be affected by the strength of this bond. The
stronger the hydrogen bond, the more upfield the PyR
resonance. This is what is seen here; for example, for
MeCo(dmgH)2Py PyR is at 8.25 ppm and O-H‚‚‚O at
18.21 ppm and for MeCo(dmestgH)2Py PyR is at 8.66
ppm and O-H‚‚‚O at 18.94 ppm.
However, a reverse and unexpected trend is noticed
when we compare the coordination shift value, ∆δ(1H,
PyR), of RCo(dioxime)2Py with that of XCo(dioxime)2Py.
This consistently occurs upfield (by 0.4 ppm) in the
latter. The ring current gives conflicting information;
for instance, cobalt dxz and dyz orbitals are involved
simultaneously in the Cofaxial ligand back-bonding
and in the π system of the cobaloxime moiety. An
increase in back-bonding would produce a depopulation
of these orbitals and consequently decrease the ring
current. Since back-bonding is higher in CofX, it should
lead to deshielding of PyR. On the other hand, the
Co-NPy bond is shorter in XCo(dioxime)2Py than in
RCo(dioxime)2Py, and so PyR is closer to the metallabi-
cycle and hence should be shifted upfield.
In view of the above discussion it is clear that the
chemical shifts in PyR behave differently in organic and
inorganic cobaloximes.
Correlations. There is a good correlation between
δ(13C, CdN) and ∆δ(1H, PyR) for all of the nine com-
plexes (1-9). This suggests the presence of ring current
throughout the Co(dioxime) metallabicycle, but the
negative slope indicates that they are effected in op-
posite directions. In contrast, the relatively poor cor-
relation and the positive sign in δ(13C, CdN) with
∆δ(13C, PyR) indicates that the ring current has little
effect.
δ(13C, CdN) ) 156.68(27) - 10.43(72) (∆δ(1H, PyR))
r2 ) 0.97, esd ) 0.33
δ(13C, CdN) ) 147.83(112) + 3.74(67) (∆δ(13C, PyR))
r2 ) 0.82, esd ) 0.77
A good correlation between ∆δ(13C, CdN dmestgH)
and the values for other dioximes is seen, showing that
the total field effect of dmestgH complexes is similar to
that of the cobaloximes with other dioximes.
∆δ(13C, CdN dmestgH) ) 2.70(12) + 1.14(3)
(∆δ(13C, CdN dpgH))
r2 ) 0.99, esd ) 0.14
Can this be due to cobalt anisotropy? One would
expect PyR to appear more downfield in XCo(dioxime)2Py
than in RCo(dioxime)2Py because of greater cobalt
anisotropy in the former.
∆δ(13C, CdN dmestgH) ) 6.01(74) + 1.6(17)
(∆δ(13C, CdN dmgH))
r2 ) 0.94, esd ) 0.48
The recent 15N NMR studies have shown that the
cobalt anisotropy works differently in XCo(dmgH)2Py
and RCo(dmgH)2Py complexes.19a Also, the ab initio
calculations in ClCo(dmgH)2Py and MeCo(dmgH)2Py
have shown that the charge densities on cobalt and
pyridine nitrogens are different in these two com-
pounds (see Supporting Information). For example,
∆δ(13C, CdN dmestgH) ) 6.90(57) + 1.24(9)
(∆δ(13C, CdN gH))
r2 ) 0.97, esd ) 0.34
UV-Vis Studies. Solid-state properties were studied
from the crystal structure of the dimesitylcobaloxime.
The solution behavior of these complexes, though well
characterized from NMR, also gets support from the
(19) (a) Schurko, R. W.; Wasylishen R. E. J. Phys. Chem. A 2000,
104, 3410. (b) LaRossa, R. A.; Brown, T. L. J. Am. Chem. Soc. 1974,
96, 2072.

Cobaloximes with Dimesitylglyoxime
Organometallics, Vol. 24, No. 7, 2005 1507
Table 5. CV Data for 1, 4, and 6 in DCM with TBAPF6 at 0.2 mV/s at 25 °C
Co(III)/Co(II)
Co(II)/Co(I)
Co(IV)/Co(III)
Epc (V, vs Epc (V, vs
Epc (V, vs E1/2 (V, vs
E1/2 (V, vs E1/2 (V, vs
ipc (µA) Ag/AgCl)
Fc/Fc+)
no. Ag/AgCl)
Fc/Fc+)
ipc (µA) Ag/AgCl)
Fc/Fc+)
∆Ep (mV) ipc (µA) ipa (µA) ipa/ipc
1
4
6
-0.57
-0.73
-0.68
-1.08
-1.24
-1.19
-1.01
-1.01
-1.01
-1.51
-1.51
-1.51
15
23
170
1.06
1.03
1.33
0.55
0.52
0.82
120
130
110
148
121
170
154
120
210
1.04
0.99
1.23
160
Table 6. CV Data for ClCo(L)2Py in DCM with TBAPF6 at 0.2 mV/s at 25 °C
Co(III)/Co(II) Co(II)/Co(I) Co(IV)/Co(III)
Epc (V, vs Ag/AgCl)
E1/2 (V, vs
E1/2 (V, vs Ag/AgCl)
E1/2 (V, vs
E1/2 (V, Ag/AgCl)
(∆Ep (mV))
E1/2 (V, vs
(∆Ep (mV))
Fc/Fc+)
(∆Ep (mV))
Fc/Fc+)
Fc/Fc+)
ClCo(gH)2Pya
ClCo(dmgH)2Py
ClCo(dpgH)2Pya
ClCo(dmestgH)2Py
-0.39 (240)
-0.66 irrev
-0.50 irrev
-0.68 irrev
-0.90
-1.17
-1.01
-1.19
-0.66 (113)
-1.12 (200)
-0.85 (105)
-1.01 (120)
-1.17
-1.63
-1.36
-1.52
1.20 (190)
1.22 (200)
1.33 (120)
0.69
0.71
0.82
a Values are in acetonitrile.
UV-vis studies. In general, the spectra of cobaloximes
are recorded in solvents such as chloroform, dichloro-
methane, ethanol, methanol, and methanol-water mix-
ture and organocobaloximes show a 5-/6-coordination7,20a
equilibrium in solvents such as methanol and ethanol
but exists as 6-coordinated complexes in chloroform and
dichloromethane. Here, we have also carried out UV-
vis studies in different solvent systems. The solvent has
little effect on the Co-C CT band but the Co-dioxime
CT band is affected significantly. For example, the
Co-C CT band in alkyl complexes 1-5 contains two
humps at around ∼380-390 nm (log ꢀ 3.60) and 420-
460 nm (log ꢀ 3.30) and do not show any deviation with
change in solvent. On the other hand, the MLCT band
in 1-9 appears at around 214-220 nm in methanol/
dichloromethane (25/1), at 236-240 nm in dichloro-
methane, and at 234-250 nm in chloroform. This is due
to more stabilization of the ground state in the cobalt
dioxime moiety in polar solvent.20b
Cyclic Voltammetry. In the cyclic voltammogram
of cobaloximes, (R/X)Co(dmgH)2B, we expect three types
of redox couples: Co(III)/Co(II), Co(II)/Co(I), and Co(IV)/
Co(III).21 Very little work has been reported on the CV
studies in cobaloximes, and hence, there is a lack of
information on these three redox systems. Also, suf-
ficient data are not available to correlate and generalize
the electrochemical behavior of these complexes. It is
true that it is difficult to get good CV data in organo-
cobaloximes, but the inorganic cobaloximes give better
cyclic voltammograms. No study has reported the values
for all three redox systems. In most of the reported work,
only the reduction process has been discussed.9a,21a,b
Part of the difficulty may lie in the decreased solubility
of these complexes in the proper solvent system. The
other associated problem is the change in coordination
number of cobalt during the reduction/oxidation pro-
cess.21 The electrochemical parameters reflect even
small modifications of the electronic properties of the
complexes brought about by the axial and equatorial
Figure 1. Cyclic voltammograms of 6 (A) and ClCo-
(dmgH)2Py (B) in CH2Cl2 with 0.1 M (NBun )PF6 as
4
supporting electrolyte at 0.2 V s-1 at 25 °C.
ligand changes. Only a few reports have appeared on
the oxidation process, and these too have been for
aquacobaloximes.21c,d Most of the reported studies have
dealt with the variation in axial ligands only, and very
little work has been done on cobaloximes with other
dioximes. CV data for 1, 4, and 6 are given in Table 5,
and CV data for ClCo(L)2Py species are given in Table
6.
The cyclic voltammogram of 6 (Figure 1A) shows two
completely irreversible waves in the reductive half, at
-0.68 and -0.96 V corresponding to Co(III)/Co(II) and
Co(II)/Co(I), respectively. On the oxidation half only one
reversible wave corresponding to Co(IV)/Co(III) (+1.33
V) is observed. On comparison of these values with those
for the other cobaloximes (gH, dmgH, dpgH), 6 is more
difficult to reduce from Co(III) to Co(II) and from Co(II)
to Co(I).
(a) Alkyl Cobaloximes. The cyclic voltammograms
for 1 and 4 are shown in Figure 2. Due to the large peak
height at the oxidation half, the reductive half is very
small or is hidden inside the background current. Two
sequential one-electron reductions (irreversible), Co(III)/
Co(II) and Co(II)/Co(I), and a one-electron oxidation,
Co(IV)/Co(III) (quasi-reversible), are observed. 1 is
easier to reduce as compared to 4. This is in line with
the higher electron donation ability of the butyl as
compared to the methyl group.
(20) (a) Marzilli, L. G.; Summers, M. F.; Bresciani-Pahor, N.;
Zangrando, E.; Charland, J.-P.; Randaccio, L. J. Am. Chem. Soc. 1985,
107, 6880. (b) Bag, B.; Bharadwaj, P. K. Inorg. Chem. 2004, 43, 4626.
(21) (a) Elliott, C. M.; Hershenhart, E.; Finke, R. G.; Smith, B. L.
J. Am. Chem. Soc. 1981, 103, 5558. (b) Alexander, V.; Ramanujum, V.
V. Inorg. Chim. Acta 1989, 156, 125. (c) Asaro, F.; Dreos, R.; Nardin,
G.; Pellizer, G.; Peressini, S.; Randaccio, L.; Siega, P.; Trauzher, G.;
Tavagnacco, C. J. Organomet. Chem. 2000, 601, 114. (d) Ngameni, E.;
Ngoune, J.; Nassi, A.; Belombe, M. M.; Roux, R. Electrochim. Acta 1996,
41, 2571.
The Co(II)/Co(I) potential should be affected only by
the dioxime moiety, since there is no axial ligation in

1508 Organometallics, Vol. 24, No. 7, 2005
Mandal and Gupta
oxidation to Co(IV) and are found to be stable, except
for the gH complex.
X-ray Crystallographic Studies. Description of
the Structures of 1, 6, and 7. A slow evaporation of
solvent from the solution of complexes 1, 6, and 7
(CH2Cl2/acetonitrile) in the refrigerator resulted in
the formation of orange crystals. The X-ray analysis of
these crystals showed the compositions as [MeCo-
(dmestgH)2Py]‚CH2Cl2, [ClCo(dmestgH)2Py]‚CH3CN, and
[BrCo(dmestgH)2Py]‚CH2Cl2, respectively. The “Dia-
mond” diagrams of the molecular structures of 1, 6, and
7 along with selected numbering schemes are shown in
Figures 3-5, respectively. Selected bond lengths, bond
angles, and structural parameters are given in Table 7
and are compared with those of related cobaloximes
(Supporting Information).
The geometry around the central cobalt atom is
distorted octahedral with four nitrogen atoms of the
dioxime in the equatorial plane and pyridine and methyl
(or Cl or Br) axially coordinated. The deviations of the
cobalt atoms from the mean equatorial N4 plane are
-0.0177(4), -0.0095(3), and -0.0130(3) Å. The deviation
is toward the axial R/X group. In contrast, the deviation
is always toward pyridine in cobaloximes with other
dioximes (gH, dmgH, dpgH) and also in Costa type
complexes.
Figure 2. Cyclic voltammograms of 1 (A) and 4 (B) in
CH2Cl2 with 0.1 M TBAPF6 as supporting electrolyte at
0.2 V s-1 at 25 °C.
Co(I). This is what we observe. The gH complex is the
easiest to reduce and dmestgH complex is the most
difficult to reduce from Co(II) to Co(I).
The structural studies on cobaloximes have focused
mainly on five points:2c (a) the axial Co-N bond length,
(b) the Co-C bond length, (c) the puckering of the
equatorial dioxime ligand, i.e., the butterfly bending
angle (R), (d) the torsion angle between the axial base
pyridine and equatorial ligand, and (e) the deviation of
the cobalt atom from the mean equatorial N4 plane. The
methyl at the 2-position in the mesityl group is close to
the pyridine ring, which causes pyridine to bend in the
solid state.
The Co-C/Cl/Br bond distances (2.002(3), 2.2243(15),
2.3396(12) Å) and Co-N5 bond distances (2.086(3),
1.978(3), 1.986(4) Å) in 1, 6, and 7 do not differ
significantly from the reported values for the corre-
sponding Me/XCo(dioxime)2Py cobaloximes.22,23
The butterfly bending angle in 1 is 7.25° and is much
larger than the reported value for MeCo(dioxime)2Py
complexes (dpgH, 4.7°; dmgH, 3.2°; gH, 2.0°). The
corresponding values in 6 and 7 are also large (6.64 and
The irreversibility can be attributed to two reasons:
(a) the slow rate of the Co(III)/Co(II) electrode reaction
so that the initial Co(III) complex and the reduced Co(II)
complex are not in equilibrium with each other at the
electrode surface and/or (b) the electrochemically gener-
ated Co(II) complex does not get recycled electrochemi-
cally to give the original Co(III) complex.
(b) Co(IV)/Co(III). Though the ratio ipa/ipc and ∆Ep
are slightly higher than that of a quasi-reversible one-
electron process in 1, 4, and 6, there is an increase in
∆Ep with increasing scan rate, indicating that the
electron transfer is quasi-reversible and consequently
no bond breaking is presumed to be occurring during
the course of oxidation. This is further substantiated
by the dependence of the Co(IV)/Co(III) E1/2 values on
the axial alkyl group or halide. As indicated by ipa/ipc
) 1 for all of the complexes, the oxidized products ((R/
X)CoIV(dmestgH)2Py are stable. Complexes with other
dioximes such as dmgH, dpgH, and gH also undergo
Figure 3. Structure of MeCo(dmestgH)2Py (1).

Cobaloximes with Dimesitylglyoxime
Organometallics, Vol. 24, No. 7, 2005 1509
Figure 4. Structure of ClCo(dmestgH)2Py (6).
Figure 5. Structure of BrCo(dmestgH)2Py (7).
Table 7. Selected Bond Lengths and Angles and
Structural Data for 1, 6, and 7
angles are much smaller than in the corresponding
cobaloximes with other dioximes. Pyridine is inclined
toward one dioxime wing (C43-N5-Co1) by 172.35,
172.59, and 172.08°, and this bending of the pyridine
ring is unusual and unique. The large bending angle
and very low twist angle must be playing key roles in
making pyridine inclined. Also, the plane of the pyridine
ring lies below one of the N-O bonds of the dioxime in
all three of the complexes. Its greater interaction with
one side of the dioxime wing as compared to that with
the other dioxime wing has resulted in a larger value
of R. The Co-N5 bond distance is longer in 1 as
compared to those in 6 and 7, indicating a greater trans
influence of the methyl group.
1
6
7
Co-C46 (Å)
Co-Cl/Br (Å)
Co-N5 (Å)
C-Co-N(ax) (deg)
O1-O4 (Å)
O2-O3 (Å)
d (Å)
2.002(3)
2.2243(15)
1.978(3)
177.99(12)
2.480(4)
2.478(4)
-0.009(6)
6.57
2.3396(12)
1.986(4)
177.67(10)
2.478(4)
2.478(4)
-0.014(1)
6.18
2.085(3)
178.33(11)
2.493(1)
2.484(1)
-0.0177(4)
7.25
R (deg)
τ (twist) (deg)
67.09
69.50
70.37
6.18°) compared to the reported values for XCo-
(dioxime)2Py. The positive value of R indicates the
bending toward the axial R/X group.
Pyridine, attached to cobalt, has twist angles of 69.50,
70.37, and 67.09° in 1, 6, and 7, respectively, and these
The dihedral angles27 increase in the order (deg) 4.7
> 3.2 ≈ 3.2 > 2.0. Can this be taken as a measure of
the cis influence of the dioxime? If so, then the cis
influence order becomes dmestgH > dpgH > chgH g
dmgH g gH and this matches the order observed in the
1H and 13C studies.
To summarize, dmestgH complexes behave differ-
ently from the reported cobaloximes; for example, the
pyridine twist angle is small and the displacement of
the cobalt atom from N4 plane is negligible and is toward
the axial methyl group, unlike in other cobaloximes,
where it is toward pyridine. The higher steric cis effect
(22) (a) Bresciani-Pahor, N.; Randaccio, L.; Zangrando, E.; Toscano,
P. J. Inorg. Chim. Acta 1985, 96, 193. (b) Parker, W. O.; Zangrando,
E.; Bresciani-Pahor, N.; Randaccio, L.; Marzilli, L. G. Inorg. Chem.
1985, 24, 3908.
(23) (a) Lo´pez, C.; Alvarez, S.; Aguilo, M.; Solans, X.; Font-Altaba,
M. Inorg. Chim. Acta 1987, 127, 153. (b) Geremia, S.; Dreos, R.;
Randaccio, L.; Tauzher, G.; Antolini, L. Inorg. Chim. Acta 1994, 216,
125.
(24) Chloro(pyridine)bis(2,3-dimesitylglyoxal
balt(III).
dioximato-κ4N)co-
(25) Cobalt anisotropy is the total field effect of the CoC4N4 system.
The field effect is the combination of the inductive effect of cobalt and
the effect of donation through Cofdioxime and Cofaxial ligand and
back-donation.
(26) Instead of δ(13C, CdN), the ∆δ(13C, CdN) value has been taken.
This is to avoid the direct effect of substituent on δ(CdN) value. ∆δ(13C,
CdN) represents the field effect.
(27) The dihedral angle (butterfly bending angle) is the angle
between two dioxime planes: i.e., the O1N1C1C2N2O2 plane and the
O3N3C21C22N4O4 plane.

1510 Organometallics, Vol. 24, No. 7, 2005
Mandal and Gupta
of dioxime with pyridine as compared to that with the
methyl group in MeCo(dioxime)2Py complexes leads to
the deviation of cobalt toward pyridine. However, in
dmestgH complexes, the C-H π interaction between the
pyridine ring and the methyl group of dimesityl ligand
forces the cobalt to deviate toward the axial methyl
group.
All three of the crystal structures show that the
cobaloxime units propagate as two-dimensional layers
through very weak intermolecular C-H‚‚‚O hydrogen
bonding and C-H‚‚‚π (Supporting Information).
ring current throughout the Co(dioxime) metallabicycle,
and the negative slope indicates that they are effected
in opposite directions. Among all the reported co-
baloximes, the dimesitylglyoxime complex is the most
difficult to reduce from Co(II) to Co(I).
Acknowledgment. The work has been supported by
a grant from the DST and CSIR, New Delhi, India. D.M.
thanks IIT Kanpur for an SRF fellowship. We thank
Dr. Ashutosh Pandey for his help in synthesizing a few
compounds.
Conclusion
Supporting Information Available: Tables and figures
containing coordination shift values for pyridine in 1-9 and
other dioximes (1H and 13C NMR) and two-dimensional layer
C-H-π and π-π interactions and CIF files for the crystal
structures of 1, 6 and 7. This material is available free of
charge via the Internet at http://pubs.acs.org. The CIF files
have also been deposited with the Cambridge Crystallographic
Data Centre; CCDC number for 1, 6, and 7 are 251396,
251397, and 251398, respectively. Copies of the data
can be obtained free of charge from the Director, CCDC, 12
Union Road, Cambridge CB2 1EX, U.K. (fax, +44-1223-
336033; e-mail, deposit@ccdc.cam.ac.uk; web, http://
www.ccdc.cam.ac.uk/).
The spectral correlations are understood much better
when both cobalt anisotropy and ring current are
considered to operate together. Dimesitylglyoxime com-
plexes are found to have the maximum cis influence
among all the reported cobaloximes. 13C NMR gives
better information than 1H NMR. The cis influence
order, on the basis of 1H and 13C studies, is dmestgH >
dpgH > chgH g dmgH g gH. The dihedral angle also
gives the same order. However, a change in the equato-
rial ligand field does not lead to any significant change
in the Co-C bond length. A good correlation between
δ(13C, CdN) and ∆δ(1H, PyR) suggests the presence of
OM049096M